In recent years, it has become increasingly important to reduce or eliminate sulfur dioxide from gases or smoke being introduced into the atmosphere. Recent widespread attention, especially in the United States and Canada, has been directed toward the environmental impact attendant on the sulfur dioxide stack emissions resulting from smelting of mineral ore, such as nickel sulfides.
Both the United States and the Canadian environmental agencies have put limitations on the amounts of sulfur dioxide stack emissions that are allowed from the stacks at mineral ore smelting locations as well as to the fugitive emissions of sulfur dioxide which are permitted into the work environment. These limitations are on a continuously more stringent basis. It is evident from the following discussion that none of the existing processes can meet these regulations without incurring undue capital expenditures.
Pyrometallurgical processing of nickel sulfide concentrates is currently carried out by the following alternative routes: roast-reverb smelting-converting; roast-electric furnace smelting-converting and flash smelt-converting. In the first two alternatives, the furnace matte grade is controlled by the degree of roasting, that is, the amount of sulfur eliminated in the roaster as sulfur dioxide, so as to minimize the amount of converting work required, but without exceeding a matte grade that could result in excessive base metal losses in the discard slag. This approach permits recycling the converter slag to the smelting furnace for recovery of metal values. In the flash smelting-converting route, a much higher oxidation of the concentrate takes place during smelting than in the above combined roast-smelting operations, thus resulting in much higher matte grade. However, the base metal losses in the furnace slag are also considerably higher and, accordingly, this slag, together with the converter slag, has to be processed for metal recovery normally in an electric furnace.
A common feature of the above-mentioned processes is the production in the smelting furnaces of mattes with sulfur contents close to the stoichiometric requirements of the base metals and the iron. Therefore, during the converting stage both iron and sulfur oxidation have to occur in order to obtain the low-iron sulfur-deficient matte which is the usual product of nickel sulfide pyrometallurgical processing.
Substantial evolution of sulfur dioxide occurs in each one of the unit operations comprising these routes, the only exception being electric furnace slag cleaning. Only for fluid bed roasting and flash smelting is the sulfur dioxide contained in a continuous high strength gas, which permits efficient fixation of the sulfur dioxide as sulfuric acid. Reverberatory smelting produces large volumes of gas containing only 1% to 2% SO.sub.2. The electric furnace gas volume is usually much smaller, depending on air infiltration, but its sulfur dioxide concentration is still well below the optimum of about 10% that can be taken by modern sulfuric acid plants. Converter gases are normally weak, unless capital intensive tight water-cooled hoods are provided to decrease air infiltration, in which case a gas stream with up to about 6% sulfur dioxide can be delivered to an acid plant. However, because of the batch nature of converting, the volume of this gas stream suffers dramatic fluctuations, and thus metallurgical acid plants require peak capacities far greater than normally used. Converting is also the major source of fugitive emissions into the work environment. Each time that a converter has to be taken off tuyeres for charging or discharging molten materials, substantial amounts of sulfur dioxide are put into the smelter atmosphere. The partial solution of this problem again requires large capital expense in providing double hoods and fans.
Accordingly, the need exists to efficiently, yet simply and without incurring excessive expense, provide a smelting process that permits a drastic reduction in sulfur dioxide emissions up the stack and in the work environment, and at the same time results in improved metal recovery without the need of special slag cleaning facilities. The present invention accomplishes just that end.